New measurements of core density collapse dynamics in the Large Helical Device

The core density collapse (CDC) is a sudden loss of plasma beta that affects super dense core (SDC) plasmas in the Large Helical Device (LHD). The CDC constrains achievable beta in LHD and may represent a more general beta and/or density limit for stellarators. MHD modeling points to destabilization of high-n ballooning modes as a trigger for collapse. The CDC’s high density and sub-millisecond timescale make it challenging to measure the event’s dynamics. Novel measurements using a 20 kHz Thomson scattering system reveal rapid event dynamics that include a narrow, short-lived core electron temperature feature, and narrowing of the density profile just before collapse. Observation is also reported of a magnetic precursor, coherent with previously-observed line-integrated density fluctuations that precede CDC by up to 1 ms. This work expands the known set of phenomena associated with CDCs and may illuminate how the edge instability couples to the plasma core and triggers its collapse.